Sendak gained international acclaim after writing and illustrating Where the Wild Things Are. The book's depictions of fanged monsters concerned some parents when it was first released, as his characters were somewhat grotesque in appearance. Before Where the Wild Things Are, Sendak was best known for illustrating Else Holmelund Minarik's Little Bear series of books.
When Sendak saw a manuscript of Zlateh the Goat, the first children’s story by Isaac Bashevis Singer, on the desk of an editor at Harper & Row, he offered to illustrate the book. It was first published in 1966 and received a Newbery Award. Sendak was delighted and enthusiastic about the collaboration. He once wryly remarked that his parents were finally impressed by their youngest child when he collaborated with Singer.
His book In the Night Kitchen, first published in 1970, has often been subjected to censorship for its drawings of a young boy prancing naked through the story. The book has been challenged in several American states including Illinois, New Jersey, Minnesota, and Texas. In the Night Kitchen regularly appears on the American Library Association's list of "frequently challenged and banned books." It was listed number 21 on the "100 Most Frequently Challenged Books of 1990-1999."
His 1981 book Outside, Over There is the story of a girl, Ida, and her sibling jealousy and responsibility. Her father is away and so Ida is left to watch her baby sister, much to her dismay. Her sister is kidnapped by goblins and Ida must go off on a magical adventure to rescue her. At first, she's not really eager to get her sister and nearly passes her sister right by when she becomes absorbed in the magic of the quest. In the end, she rescues her baby sister, destroys the goblins, and returns home committed to caring for her sister until her father returns home.
Sendak was an early member of the National Board of Advisors of the Children's Television Workshop during the development stages of the television series Sesame Street. He also adapted his book Bumble Ardy into an animated sequence for the series, with Jim Henson as the voice of Bumble Ardy. He wrote and designed three other animated stories for the series: "Seven Monsters" (which never aired), "Up & Down", and "Broom Adventures".
Sendak produced an animated television production based on his work titled Really Rosie, featuring the voice of Carole King, which was broadcast in 1975 and is available on video (usually as part of video compilations of his work). An album of the songs was also produced. He contributed the opening segment to Simple Gifts, a Christmas collection of six animated shorts shown on PBS TV in 1977 and later issued on VHS in 1993. He adapted his book Where the Wild Things Are for the stage in 1979. Additionally, he has designed sets for many operas and ballets, including the award-winning (1983) Pacific Northwest Ballet production of Tchaikovsky's The Nutcracker, Houston Grand Opera's productions of Mozart's The Magic Flute (1981) and Humperdinck's Hansel and Gretel (1997), Los Angeles County Music Center's 1990 production of Mozart's Idomeneo, and the New York City Opera's 1981 production of Janáček's The Cunning Little Vixen.
In the 1990s, Sendak approached playwright Tony Kushner to write a new English version of the Czech composer Hans Krása's children's opera Brundibar. Kushner wrote the text for Sendak's illustrated book of the same name, published in 2003. The book was named one of the New York Times Book Review's 10 Best Illustrated Books of 2003.
In 2003, Chicago Opera Theatre produced Sendak and Kushner's adaptation of Brundibar. In 2005, Berkeley Repertory Theatre, in collaboration with Yale Repertory Theatre and Broadway's New Victory Theater, produced a substantially reworked version of the Sendak-Kushner adaptation.
He also created the children's television program Seven Little Monsters.